Stephen King’s novel “It” is one of his mammoth fantasies, a heavyweight doorstop with a brilliant idea. What if, 27 years after defeating ultimate evil in their small town, a group of childhood heroes was called back into service?

The 2017 hit adapted from the book wisely focused on the novel’s first half, when the 13-year-old members of the “Losers Club” of Derry, Maine, went to battle against a malevolent shape-shifting clown called Pennywise (played by Bill Skarsgard).

Now comes the sequel, “IT Chapter Two,” in which the adult versions of the characters reunite. In true Stephen King fashion, the movie runs an exhausting 169 minutes. It’s big, it’s bombastic, and it misses something crucial about the appeal of King’s work.

Most of the Losers have escaped Derry in the intervening decades. Because of some supernatural witchery, they’ve forgotten their experiences — and their pledge to return if Pennywise every shows his red nose around the place again. Only Mike (Isaiah Mustafa) stayed in town, and he summons the others.

In a highly dubious bit of mumbo-jumbo, each Loser must explore Derry alone, in search of a token from the past. This means the middle hour of the film tracks each character in a standalone adventure, which robs us of the fun comradeship of the group.

Part of the movie’s bloat comes from flashbacks to the young Losers. You can understand why they’re here — the young cast is more appealing than the adult cast — but the actual material contained in the flashbacks is almost entirely unnecessary.

The film also begins with a baffling sequence of gay-bashing, which gets Pennywise onstage but leads absolutely nowhere. Somewhere there’s probably a four-hour cut of this thing that ties up the loose ends.

I’m glad this cut found time for Stephen King’s acting cameo. He plays a crafty old coot, and his five-minute performance is spot-on.

Director Andy Muschietti, returning to duty, throws most of the special-effects budget into the climactic showdown with Pennywise. Substituting breakneck editing and migraine-inducing strobe lights for actual ideas, Muschietti wants to convince you that something exciting is happening.

I was not convinced. But up to that point, “IT Chapter Two” is enjoyable enough, especially with Hader’s one-liners lightening up the mood — actually, the movie is pretty jokey overall, considering the child-killing epidemic going on.

With all its moving pieces, the movie has no time to capture the pull of Stephen King’s original idea. Putting aside the ridiculous clown and the various knife-wielding psychos, there’s something powerful about the concept of childhood damage that forms a bond between people, a bond that time cannot weaken.

King has always been good at getting that kind of emotional ballast into his stories. “IT Chapter Two” is more interested in being a funhouse ride, all jumping monsters and flashing lights. The Losers deserved better.

“IT Chapter Two” (2 stars)

The sequel to the 2017 hit covers the second half of Stephen King’s novel, when the members of the Losers Club are called back to Derry, Maine, after 27 years, to do battle once again with the evil clown called Pennywise (Bill Skarsgard). The movie’s enjoyable enough until the bombastic climax, but ultimately it substitutes noise and flashing lights for the emotional ballast of King’s ideas. With Jessica Chastain, James McAvoy.